Abstract: In recent years, reversible excited-state photoacids have been exploited for high spatiotemporal sensing of local pH during electrochemical reactions, to study proton-coupled reaction mechanisms, to trigger processes in biological organisms relevant to energy transduction, and in the design of artificial light-driven proton pumps. Successful application of these photoacids in various systems requires non-trivial experimental quantification of accurate excited-state Brønsted-Lowry acidities (thermodynamics) and excited-state proton-transfer rate constants